FMCW radar sensors are used in motor vehicles for detecting the traffic environment, in particular for finding the position of other vehicles. The position-finding results may be utilized for various assistance functions, for example, for automatic distance control, automatic collision warning, or automatic triggering of an emergency braking operation when there is the risk of an imminent collision.
The frequency of the baseband signal corresponds to the frequency difference between the signal that is transmitted at a given point in time and the signal that is received at the same point in time. Due to the frequency modulation of the transmission signal, this frequency difference is a function of the propagation time of the signal from the radar sensor, to the object, and back, and thus is a function of the distance from the object. However, due to the Doppler effect, the frequency difference also includes a component that is determined by the relative speed of the object. Therefore, measuring the frequency difference on an individual ramp still does not allow a determination of the distance and the relative speed, but, rather, yields only a linear relationship between these variables. This relationship may be represented in a distance/speed diagram (R-v diagram) as a straight line.
To obtain unique values for the distance and the relative speed, in one common type of FMCW radar, operations are carried out having frequency ramps which rise and fall in alternation. In the R-v diagram a different straight line is then obtained for each ramp, and the distance and the relative speed of the object are given by the intersection point of these two straight lines.
In another specific embodiment, operations are carried out with a sequence of identical, relatively short frequency ramps, so-called “rapid chirps,” which have a high frequency deviation in relation to their duration, and therefore are so steep that the distance-dependent component dominates in the baseband signal, while the Doppler component represents only a small correction. This correction is determined by tracking the phase change of the baseband signal from ramp to ramp. Use is thus made of the fact that the phase of the baseband signal responds relatively sensitively to the slight change in the distance from the object, which results from the relative movement of the object from one frequency ramp to the next during the short time interval.
However, since the phase change is a periodic function of the relative speed, the relative speed may be unambiguously determined only when it is so small that the phase change is less than one-half period (i.e., less than π).
The radar sensor generally includes multiple antenna elements which are spaced apart from one another on a line, for example a horizontal, so that different azimuth angles of the located objects result in differences in the run lengths which the radar signals have to cover from the object to the particular antenna element. These run length differences result in corresponding differences in the phase of the signals which are received from the antenna elements and evaluated in the associated evaluation channels. The incidence angle of the radar signal, and thus the azimuth angle of the located object, may then be determined by comparing the (complex) amplitudes received in the various channels to corresponding amplitudes in an antenna diagram.
In a multiple input/multiple output (MIMO) radar, a higher angular resolution capability is achieved by operating not only with multiple receiving antenna elements, but also with multiple transmitting antenna elements, whereby different combinations of transmitting antenna elements and receiving antenna elements are evaluated. The varying positions of the transmitting antenna elements then result in additional phase differences, and thus, in signals which are equivalent to signals that would be obtained with a configuration using a single transmitting antenna element and additional (virtual) receiving antenna elements. In this way, the aperture is virtually enlarged and the angular resolution is thus improved.
In a MIMO radar, the various transmission signals must be orthogonal with respect to one another. This may be achieved, for example, by code multiplex, frequency multiplex, or time multiplex. However, the code multiplex method requires a high level of effort, and allows only limited orthogonality of the signals. In the frequency multiplex method, there is the disadvantage that the phase and the Doppler shift are a function of the particular wavelength. The method provided here is therefore based on the time multiplex principle. However, there is the problem that relative movements of the located objects in conjunction with the time offset between the various switching states result in phase differences which make the subsequent angular estimation more difficult. One option for compensating for these phase shifts is to estimate the phase shifts caused by the object movements, based on the measured relative speeds; however, this is possible with only limited accuracy.